(a) Field of the Invention
The present invention relates generally to a synchronous rectifying device and method, and more particularly but not exclusively to a synchronous rectifying device and method applicable to a resonant converter.
(b) Description of the Related Art
A conventional converter for outputting a DC voltage uses a diode rectifier. However, a conduction loss of the diode rectifier causes a power loss of the converter. When the diode is conducted to flow the current, a conduction loss is generated by the forward-voltage drop of the diode and the flowing current.
In order to reduce the conduction loss on the secondary side, the converter can use synchronous rectifier network unit using a switch such as a MOSFET. The conventional converter uses a winding voltage at the secondary coil of the transformer configuring a converter for generating a gate signal for controlling the switch.
However, in the case of the resonant converter, it is difficult to control the synchronous rectifier network unit switching by using the winding voltage at the secondary coil of the transformer. In the case of an LLC resonant converter including a resonance inductor, a resonance capacitor, and a plurality of power switches, while the power switches are turned on, the current having a resonance frequency determined by the resonance inductor and the resonance capacitor flows to the power switches. When the current flowing through the power switches becomes less than the current flowing through a magnetizing inductor configuring the resonance inductor, and during the dead time in which the power switches are turned off, no current flows to the transformer secondary coil, and a voltage with ripple is generated at the secondary coil of the transformer by resonance. Further, to prevent the output capacitor from being discharged, the synchronous rectifying device has to be turned off before the current flowing to the synchronous rectifying device becomes 0. Hence, it is difficult to generate a control signal for controlling the synchronous rectifier network unit switch stably.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.